The estimation of standing crop is important in the management of rangeland resources. Direct measurements by clipping, drying, and weighing of herbaceous vegetation are time-consuming and labor-intensive. Therefore, non-destructive methods for efficiently and accurately estimating standing crop are needed in rangeland forage management. We assessed a visual obstruction (VO) technique to estimate standing crop (SC) of northwest native bunchgrass communities at The Nature Conservancy’s Zumwalt Prairie Preserve in northeastern Oregon. This method involves obtaining a height-density index by measuring the height of a pole that is obscured by vegetation when viewed from the side. Five hundred seventy six plots (0.5 m2) were subjected to VO measurement; and subsequently, all vegetation within a plot was clipped to ground level. Only current year’s crop was taken. Regression analysis was used to evaluate the relationships of VO to standing crop, with standing crop as the dependent variable. Total standing crop was 1261 ± 51 kg∙ha-1 and mean of VO measurement was 12.8 ± 0.4 cm for vegetation in the study site. By growth habit of plants, standing crops were 688 ± 26, 13 ± 26, 416 ± 26, and 144 ± 26 kg∙ha-1 for grasses, grasslikes, forbs, and shrubs, respectively, and all growth habits differed from each other (P < 0.01). A positive (P < 0.01) linear relationship occurred between VO and SC measurements, however, correlation was low with only 46% of the variation in standing crop being attributable to VO (y, kg∙ha-1 = 270.58 + 77.66x, cm; r2 = 0.46, n = 576). In heterogeneous mid-height bunchgrass communities like the Zumwalt Prairie Preserve, the VO technique will not accurately predict standing crop although many wildlife investigators will still find it useful for describing vegetative structure in these communities. Consequently, we recommend that, if considering VO as a surrogate for SC, investigators should calibrate VO technique against clip plots to evaluate applicability to their situation.

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The project was supported by the National Research Initiative of the USDA Cooperative State Research, Education and Extension Service, grant number 2006-35101-16572, The Nature Conservancy and Oregon State University’s Eastern Oregon Agricultural Research Center.